The Prince William County Board of Supervisors, after intense behind-the-scenes negotiating, elected Edwin C. King, a Democrat from Dumfries, yesterday to serve as its chairman.

King will succeed fellow Democrat G. Richard Pfitzner, who announced in November that he was stepping down after serving just one year as head of the seven-member board. Pfitzner's term had been marked by controversy over his outspoken personal style, which some said was excessively combative.

King's election, by a 5-to-2 vote that followed party lines, came just minutes into the board's first meeting of the year and was preceded by almost no discussion. However, the agreement to select King was reached only after heavy politicking among board members and prominent Prince William Democrats, according to officials involved with the negotiations.

The negotiations reversed a highly unusual agreement that apparently had been reached among some board members, before the announcement of Pfitzner's resignation, to make a Republican, Donald E. Kidwell, head of the Democratic majority board.

Many local Democrats, particularly members of Prince William's delegation to the General Assembly, strenuously objected to Kidwell's becoming chairman and lobbied to prevent it, according to Floyd C. Bagley, who retired last year as delegate from the 52nd District, and is now chairman of the Prince William Democratic Committee.

"It was a bit of temporary insanity on the part of some Democrats," Bagley said yesterday of the plan to install Kidwell. " . . . I was opposed to it from the start."

Bagley said others who acted to prevent the deal included state Sen. Charles J. Colgan, Del. David G. Brickley and Supervisor Kathleen Seefeldt, who was board chairman until a year ago, when she was ousted by Pfitzner and Kidwell, among others.

The plan to install Kidwell may have stemmed from agreements reached a year ago when Pfitzner was trying to replace Seefeldt, Bagley said.

"The whole thing smacked of some kind of political deal being made with the chairmanship as the carrot on the end of the stick," Bagley said.

One person left with no carrots to chew yesterday was Kidwell. "Don't ask me, talk to the Democrats," Kidwell said when asked about the chairmanship. "I was told by several members that I was to be chairman."